If you give somebody permission to drive your car in Spain it is recommended that they carry a document confirming you have given them your permission. A copy of this document can be obtained from the N332 website by clicking on the link below.
There is also another article about British people being legally able to drive in Spain on a UK driving licence provided it is not one which lasts for years. So many retired Brits with 3 year licences were conned into taking a Spanish test and switching to a Spanish licence and didn’t need to.